1. Technical Field
The present disclosure relates to generating a reference level, and more particularly, to a device for generating a variable slice reference level.
2. Discussion of Related Art
A conventional television set displays about 24 to 30 frames per second. According to the theory of persistence of vision, the human eye retains an image for a brief instant after it has disappeared. Interlaced television has been designed to make use of persistence of vision by not having to draw an entire image at once. For a conventional interlaced television, one frame is displayed in two scans, with the first scan drawing the odd lines of a screen and the second scan drawing the even lines of the screen. Here, one scan is called a field, and two scans including two fields are collectively called one frame.
Information stored in the field is displayed from an upper left corner to a lower right corner of the screen in a raster pattern. When the stored information in the current field is information pertaining to the odd lines of the screen, information stored in the next field is information pertaining to the even lines of the screen. The information stored in the next field is also displayed in the raster pattern from the upper left corner of the screen to the lower right corner of the screen.
In an apparatus performing a raster operation as described above, after a raster scan has finished traveling from the upper left corner of the screen to the lower right corner of the screen, the scanning beam must reset back to the upper left corner of the screen to begin the next scan. During the time interval when the location of the scanning beam changes, a signal cannot be displayed. This time interval is called a vertical blanking interval (VBI). For convenience of description, a time interval between frames will also be called the VBI.
Even though the conventional television set may not be able to display a signal during the VBI, the television set may continue to receive a broadcasting signal. The additional signal transmitted during the VBI may include various value added services such as a closed-caption television broadcasting, a teletext broadcasting service, a program code broadcasting service, and/or an interactive service using HTML can be provided. The additional data is added in the VBI region located in a middle portion of the continuous data stream to be transmitted.
Closed-caption television broadcasting originates from research conducted for the benefit of hearing-impaired people by the National Captioning Institute, which was founded by the United States Federal Government in 1979. Standard specifications for closed-caption television broadcasting and extended data services (EDS) were published as EIA-608 in April of 1993 by the Electronic Industries Alliance In a closed-caption broadcasting service provided in the U.S.A., the 21st line of the VBI and a clock frequency of 503 KHz are used for closed-caption broadcasting.
In the teletext broadcasting service, texts such as urgent news, weather information, stock information, and day-to-day information are coded into digital signals. The coded digital signals are input to the VBI region to be transmitted. The coded digital signals are decoded by a decoder of a viewer's television set and displayed on the television screen. Other than the text described above, the teletext broadcasting service can be applied to a field such as a caption broadcasting service for the hearing impaired or for displaying alternative-language subtitles,
A program code broadcasting service provides information related to television programming and a reservation recording service. In the program code broadcasting service, information such as standard time information, daily and weekly program list information, and the like, is coded into a digital signal. The digital signal is then input to the VBI region of a broadcasting signal to be transmitted. Through the program code broadcasting service, a user can access services such as a reserved recording service for a video cassette recorder, a recording data service in the case of a power outage, an automatic setting and adjustment service of the current time, a classification of broadcasting programs according to types of programs, and the like.
An interactive service using hyper-text markup language (HTML) may be transmitted during the VBI. The interactive service includes an inter-cast, an inter-text, a bit-cast, a data-wave, and the like. The inter-cast, which is a compound word meaning internet and broadcast, is a service for adding and transmitting web data in an HTML format in a conventional broadcasting program. The web data is added to the VBI region of a television broadcasting signal and transmitted. A television broadcasting program and additional information in a web data format can be viewed simultaneously by a personal computer in which a receiving card for TV broadcasting is installed. When a user of a computer is connected to the Internet, it is possible for the user to navigate to a different web site linked by the web data received by the receiving card for TV broadcasting.
FIG. 1 illustrates signals and data used in the VBI region
When closed-caption data for closed-caption television broadcasting is added in the VBI region of a television signal to be transmitted, the closed caption data is added to the 21st data line of a plurality of data lines included in the VBI region. The caption data is divided into a clock run-in interval P1, a start-up interval P2, and a caption data interval P3.
The frequency of a clock signal CLOCK of the clock run-in interval P1 is 503 kHz. A period of the start-up interval P2 is equivalent to eight times the period of a clock signal CLOCK. The associated start-up signal is synchronized with the associated clock run-in signal When a predetermined time elapses after the start-up signal is enabled, for example, at a beginning of the caption data interval P3, a logic value of the caption data is determined using a comparison reference signal having a predetermined voltage level. The voltage level of the comparison reference signal is referred to as a slice level.
A voltage difference of the caption data is about 0.7 V. Generally, the slice level is fixed to an arbitrary voltage level between a maximal voltage level and a minimal voltage level. Here, the phrase “performing a slice operation” indicates determining a logic value of the caption data by comparing a magnitude of the caption data to the slice level. For example, the caption data is determined to have a value of a logical high, when the magnitude of the caption data signal is greater than the slice level and the caption data is determined to have a value of a logical low when the magnitude of the caption data signal is equal to or less than the slice level.
In order to prevent a slice error from occurring in a case when a magnitude of a caption data signal changes due to a noise, a method of using an additional reference signal for the comparison has been proposed. However, since the two reference signals for the comparison have fixed voltage levels, it is difficult, to correctly slice the caption data when the caption data contains noise of various magnitudes.
When a teletext used in a teletext broadcasting service is input to the VBI region to be transmitted, the teletext can be divided into a bit sync interval P1, a byte sync interval P2, and a text-data interval P3,
The byte sync interval P2 has a width of eight clock signals. A byte sync signal BYTE SYNC is synchronized with a bit sync signal BIT SYNC. When a predetermined time elapses after the byte sync signal is enabled, for example, at a beginning of the text-data interval P3, a logic value of the text-data is determined using a reference signal for comparison.
In the teletext broadcasting service, since the magnitude of the reference signal for the comparison, for example, a slice level, is fixed to a predetermined voltage level, it is difficult to correctly slice the caption data when the caption data contains noise of various magnitudes.